1. Field of the Invention
The invention relates generally to circuit breakers, and, more particularly, to circuit breakers with instantaneous and delayed trip capability.
2. Description of the Prior Art
Circuit breakers provide protection to electrical circuits and apparatus by automatically interrupting load current upon occurrence of overload conditions. Normally, circuit breakers employ an inverse time-current trip characteristic, such that extreme overloads will cause almost immediate interruption and low to moderate overloads will induce a time delay before trip to allow transient conditions to clear themselves before interruption occurs, thereby preventing unnecessary power outages.
Traditional circuit breakers employed two tripping devices to provide this inverse time-current characteristic. Instantaneous trip was produced by a magnetic device wherein load current through a conductor would produce a magnetic field during high overload conditions to attract an armature and actuate a trip mechanism. Time delay trip functions were provided by a bimetal element connected to conduct load current. Under low to moderate overloads, the bimetal would heat and deflect, the deflection being dependent upon the degree of overload and the length of time during which it occurs. When the bimetal deflected past a certain point, it released a latch or otherwise actuated the trip mechanism to produce an interruption.
More sophisticated electrical distribution protection systems require time-current tripping characteristics carefully tailored for the circuit breakers involved. This is provided in some instances using current transformers disposed around the circuit conductors to provide a current signal to an electronic circuit, the parameters of which are adjusted to provide an actuating signal to the trip mechanism according to the desired time-current tripping characteristic.
Such electronic trip circuits are very successful in many applications. However, the increased component count including a large number of electronic devices increase the probability of component failure. In addition, some electronic trip circuits require sensitive permanent magnet trip actuators which can suffer damage if the circuit breaker is subjected to rough handling prior to installation.
On some applications, the cost of providing a current transformer and an electronic tripping circuit is not warranted. It would therefore be desirable to provide a simple, low cost circuit breaker trip mechanism which will give an inverse time-current tripping characteristic with fewer components.